Two U.S. senators in Pristina on Monday stated that for Kosovo to fulfill its aim of joining NATO, a peace agreement with Serbia must be implemented.

U.S. Democratic senators Gary Peters, a member of the armed services committee, and Chris Murphy, a member of the foreign relations committee, urged the two nations to promptly implement the agreement struck in March with the help of the European Union. A congressional delegation visiting the Balkans includes them.

“The pathway (for Kosovo) to NATO and to the European Union runs through an agreement with Serbia. That’s a hard fact,” Murphy told journalists at the U.S. embassy in Pristina.

Kosovo, which declared independence from Serbia in 2008, is not recognised as a state by four NATO members: Romania, Spain, Greece and Slovakia.

Murphy said the four could be convinced to accept Kosovo in NATO if differences with Serbia were settled. “It is dependent on this agreement being done and implemented,” he said.

Despite a deal in March to normalise relations, there has been no progress on the ground especially in northern Kosovo where some 50,000 Serbs still do not accept Kosovo’s statehood.

Washington is Kosovo’s main supporter, both politically and financially. There are currently around 4,000 NATO troops in Kosovo, of whom 600 are from the United States to maintain the fragile peace.

Serbia and its traditional ally Russia do not recognise Kosovo’s independence, and Moscow has blocked the country’s bid to become a member of the United Nations. Belgrade still considers Kosovo part of its territory.